20 



C. R. VAN HISE 



to his experiments. When the strata were compressed in these 

 experiments there was flowage from below the synclinoria, but 

 not a sufficient amount to allow the synclinoria to become as 

 plicated as the anticlinoria, and at the same time to be at a 

 lower level. 



3. In anticlinal mountain masses the cores, composed of 

 the oldest and originally deepest-buried strata, are often less 

 closely plicated than the strata on the flanks of the mountains, 

 which are composed of younger rocks. In estimating the crustal 

 shortening of such mountain masses, we have, therefore, the 



Fig. 7. — Possible relation of secondary folds to joints. 



difficulty already mentioned, that is, folds of different layers 

 vary in their closeness. This occurs notwithstanding the fact 

 that upon the limbs one would expect that differential move- 

 ment between the layers, or shearing, which tends to stretch 

 them rather than to produce plications, are at a maximum. The 

 greater folding of the limbs, as compared with the cores may be 

 partly explained by the principle already given, that in general 

 folds die out as depth increases, and consequently the older 

 strata are least folded. However, the plications are probably 

 to be chiefly explained by the gliding of the material down the 

 slope upon the flanks of the mountain, because under the stress 

 of gravity (Fig. 7). The strata on the crests may have been 

 removed by erosion, or, as explained in another place (see 

 p. 24) , the stresses may have there produced joints. In either 



